When wet plastic concrete mix is to be emplaced at a job site that is at some distance from a plant where the material is batched or mixed, optimum utilization of vehicles is obtained when each can quickly discharge its load and then promptly start back to the batching plant for another load. It has been recognized that such fast turn-around of the transport vehicle can be achieved if a satisfactory surge bin is located at or near the job site, into which the vehicles can dump their loads and which can in turn discharge the mix, as and when needed, into wheelbarrows, a crane bucket, a concrete pump, a paving machine or other placement means. Without a surge bin, each transport vehicle has to spend a long period at the site, discharging its load little by little into the placement means; and to maintain constant availability of mix at the job site there has to be another loaded vehicle standing by while the preceding one finishes delivering its load. With a surge bin, fewer transport vehicles are needed for placing a given amount of concrete in a given time, because each vehicle spends most of its time in the actual transportation of mix, and the surge bin--as the name implies--absorbs the surges in delivery and ensures a constant supply of mix at the job site.
Several surge bins for wet plastic concrete mix have appeared on the market from time to time, but no such prior surge bin has been completely satisfactory.
Perhaps the least expensive and least complicated concrete mix surge bin heretofore available comprises a sturdy frame which normally rests on the ground but which has retractable wheels on which the unit can be towed. Tiltably mounted on the frame is a body that has an open-topped front portion and has top, bottom and side walls at its rear portion that converge rearwardly towards a gated discharge outlet in its rear end. In a lowered position of the body, its top is low enough to permit a conventional dump truck to discharge over its front wall into its open-topped front portion. For discharge of the surge bin a hydraulic actuator lifts the front end of its body, swinging the body up about a horizontal, laterally extending axis near its rear end that is at a high enough level to allow a crane bucket or a concrete pump hopper to be located beneath the discharge outlet. A serious disadvantage of this surge bin is that it has no provision for remixing. High slump (thin consistency) concrete mix tends to segregate during transport to the surge bin, and mere discharge into the surge bin does not satisfactorily remix it High slump concrete also tends to segregate in the surge bin if held for any substantial time without remixing. Low slump concrete if held without remixing, tends to set up rather quickly and to be discharged in clumps. In this prior surge bin, problems due to the lack of a remixing agitator are reduced to some extent by small capacity, which is not much greater than that of a transport vehicle. However, the combined disadvantages of low capacity and lack of an agitator severely limit its utility, and this prior surge bin appears to have had correspondingly limited sales notwithstanding its low cost.
U.S. Pat. No. 3,612,491, issued to R. W. McKillop et al in 1971, discloses a remixing surge bin which avoids some of the disadvantages of the device just described, but has other significant disadvantages, including substantially higher cost. The patent discloses an axially elongated mixing drum rotatably mounted on a trailer chassis with its axis extending lengthwise of the chassis and inclined forwardly and downwardly. The front end of the drum is closed. At its higher rear end the drum has a large concentric opening through which it is charged and through which it also discharges. A funnel-like charging hopper directly behind the drum has a downwardly and forwardly inclined spout that extends into the drum through the opening and terminates inside the drum at about the level of the drum axis. Beneath the open rear end of the drum is a downwardly and rearwardly inclined delivery chute into which mix issues by spilling over the lip of the drum opening, driven out of the drum by a spiral blade therein that moves the mix rearward when the drum rotates in one direction. When the drum rotates in the opposite direction, the blade draws mix away from the charging hopper and drives it towards the closed front end of the drum for remixing. Since the charging hopper is at a high level, and is necessarily rather small, transport vehicles discharge their loads onto a belt conveyor which has its receiving end in front of the unit, at a low level, and which carries the material upwardly and rearwardly over the drum, discharging into the hopper. To avoid spillage, concrete mix must be discharged onto the conveyor rather slowly, and the need for such gradual unloading of transport vehicles is an undesirable requirement in a surge bin, especially since reduction of transport vehicle turn-around time is a major reason for use of a surge bin. The surge bin of McKillop et al has the further disadvantage that the mix inside the drum is not readily visible, and therefore it is inconvenient and time consuming to determine how much material is in the drum and whether or not the mix needs additional water. Another serious disadvantage is that cleaning out the drum may be difficult and very unpleasant if a person has to work inside it to chip away hardened concrete.
A prior surge bin invented by the present applicant is disclosed in U.S. Pat. No. 3,866,889. It is an adaptation of a previously devised open-topped agitating dump body already on the market, mounted on a truck chassis so that it can be used for transporting concrete mix (or other material) when not needed as a surge bin. The bottom wall of the body slopes upwardly and rearwardly to a gated discharge outlet, and the body is tiltable about a transversely extending horizontal axis that is near the discharge outlet and hence at a high level. Mix is charged into the body from a belt conveyor which has its receiving end in front of the truck at a low level and which extends upwardly and rearwardly over the truck cab to deliver into the front of the body. Inside the body is an agitator that remixes the material and/or moves it rearward towards the discharge outlet, where the gate meters outflow from the body. Concrete mix discharged from the body is distributed by a chute or conveyor that extends rearwardly from the truck chassis and onto which the body discharges. As compared with the McKillop et al device, this surge bin has the obvious advantages of being lower in cost and of having an open-topped body in which concrete mix is directly visible and which is much easier to clean than a drum. However, because it also comprises a belt conveyor, it has the same disadvantage of tying up a transport vehicle during the slow and gradual unloading that is necessary to prevent spillage at the conveyor. Furthermore, in its preferred form comprising a truck chassis, the capacity of this surge bin is limited to one truck load. Considered apart from its capability for transport use, it must be regarded as relatively expensive for a surge bin of such small capacity.
Another concrete mix surge bin that has been commercialized but has not received unqualified approval is disclosed in U.S. Pat. No. 3,901,485, to F. W. Schwing. It comprises a mixing drum resembling that of McKillop et al in being axially elongated and mounted on a trailer chassis, but the Schwing mixing drum has a gated concentric outlet at its front end in addition to a concentric inlet opening at its rear end. The axis of the Schwing drum is inclined oppositely to that of McKillop et al; that is, the axis of Schwing's drum slants upward and forward towards the discharge outlet, which is at the small end of a long, forwardly tapering frustoconical portion of the drum. Behind the drum and closely adjacent to it is a charging hopper which has its upper edge at a low enough level for transport vehicles to discharge directly into it. A spiral blade in the drum draws mix forwardly away from the charging hopper and up along the drum, for remixing or for driving the material out of the discharge outlet when its gate is open. Because the drum outlet is at a relatively high level, it can discharge directly into the feed hopper of a concrete pump or into a crane bucket. However, the charging hopper at the rear of the drum has a relatively small capacity, and mix is drawn out of it relatively slowly by the spiral blade in the drum so that the Schwing surge bin, like the others described above, compels a relatively slow unloading of each transport vehicle. Furthermore, as with the McKillop et al surge bin, the quantity and consistency of the material inside the drum is hard to see, and cleaning out the drum is difficult and time consuming.
It will be apparent that a satisfactory surge bin for wet plastic concrete mix poses several requirements which have not heretofore been satisfactorily reconciled. A very important requirement is that it be capable of receiving mix from a transport vehicle as quickly as the vehicle can dump, to ensure fast turn-around of the vehicle. A dump truck can discharge wet plastic concrete mix at rates as high as about one cubic yard per second, whereas a conventional belt conveyor can accept such concrete mix at rates no higher than about one cubic yard in 20 seconds. Hence, a conventional belt conveyor is obviously an unsatisfactory expedient for charging a surge bin. A further and complicating requirement is that the surge bin should have its outlet at a high level for discharge, so that it can feed directly into a placement means such as a wheelbarrow, a crane bucket, or the hopper of a concrete pump; but the surge bin should nevertheless be adapted to be loaded from a transport vehicle that has its discharge outlet at a low level.
Specialized dump bodies for transporting concrete mix were disclosed in U.S. Pat. Nos. 2,465,899, 2,613,106 and 2,674,489, all issued to G. Maxon Jr. Such bodies, which have had a substantial degree of commercial success, are arranged to have their outlets at a high level during discharge. Perhaps the need for a surge bin to discharge at a high level could be rather easily accommodated if the surge bin were arranged to take advantage of the high discharge feature of the dump bodies disclosed in these Maxon Jr. patents. However, the most desirable vehicle for carrying concrete mix from a mixing plant to a job site is an ordinary dump truck. Because it can be used for a wide variety of hauling jobs, a conventional dump truck can find almost constant employment, and therefore its fixed costs (license fees, insurance, etc.) can be much lower per hour of operation than those of a vehicle specialized for transporting concrete mix.
It follows that a satisfactory surge bin for wet plastic concrete mix should be arranged to be charged from a conventional dump truck. Obviously, if it can be loaded from such a truck it can also be loaded from any vehicle intended for specialized transport of concrete mix, such as a truck mixer or a truck with an agitating dump body.
If the surge bin is to be loaded from conventional dump trucks, provision for remixing in the surge bin is practically mandatory, and experience has shown that such provision is highly desirable even when the surge bin is loaded exclusively from truck mixers or agitating dump body trucks. The surge bin agitator should preferably assist in the metered discharge of mix from the surge bin; and when operating it should not interfere with charging of mix into the bin. It should be effective to agitate all of the material in the surge bin.
Another desideratum of a satisfactory surge bin is that its body have an open top, rather than being a rotatable drum. Transport vehicles can dump directly into an open-top body, making possible the quick turn-around that is a principal objective for use of a surge bin. Concrete mix in such a body is visible at all times, so that the quantity and the consistency of the material can be readily ascertained and water, as needed, can be added quickly and easily. An especially important advantage of a surge bin body that is open at its top is that it is much easier to clean than a drum.
High capacity, simplicity, extreme sturdiness and low cost are further requirements for a satisfactory surge bin that must be reconciled with one another and with the other requirements set forth hereinabove.